The present invention relates to a semiconductor device utilizing two-dimensional carriers, such as a heterojunction device or an MOS device, and, more particularly, to a field effect transistor which is formed under a gate electrode and which controls a current flowing through a split conduction channel which branches and then joins again.
Prior art field effect transistors having a heterojunction arrangement (e.g , HEMT: High Electron Mobility Transistor), are typically so designed that the density of electrons in an electron accumulation layer generated in the vicinity of the interface of the heterojunction which is formed by joining two kinds of semiconductors having different electron affinity from each other is varied by a voltage impressed on a gate electrode so as to control the electrical conductivity between two other electrodes, i.e., source and drain electrodes, as described in the Official Gazette on Japanese Patent Publication No. 53714/1984.
The above-described prior art is aimed at significantly improving electron mobility by a method wherein a semiconductor layer operating as a source of supply of electrons contributing to the electrical conductivity is separated from a semiconductor layer operating as a conduction channel by using different semiconductor layers. Concretely, an n-type AlGaAs (e.g., Al.sub.0.3 Ga.sub.0.7 As) layer doped with Si is used as the source of supply of electrons, while a non-doped or low-concentration n-type GaAs layer is used as the conduction channel so as to reduce electron dispersion.
As semiconductor elements have become more minute, it has become necessary to take the information regarding the phase of the carrier into consideration. In a field effect transistor having a size of about 0.1 .mu.m and having a construction wherein a current channel branches at one point and then joins again at another point, interference will occur due to the difference in phase between two electron waves at the joining point. There is no description, in the above-stated prior art, regarding the information on the phases of such electron waves.
A semiconductor device wherein the phase information on the electron waves is controlled by a magnetic field, on the other hand, is discussed in the Physical Review Letters, Vol. 55 (1985), pp. 2344-2347. According to the teachings of this article, the current channel is divided in two by making two heterojunctions close to each other, with a magnetic field being impressed in a direction vertical to a plane containing the current channel divided in two, so as to vary the phases of electron waves passing through the two parts of the current channel to thereby control the amplitude of a current value. This is based on the Aharonov-Bohm effect, as described in the article.
This technique presents a semiconductor element utilizing the phase information on the electron waves, suggesting a direction of future advance for minute elements However, the arrangement described in this article has problems in terms of simplicity and practicability since a magnetic field is used for controlling the phases of the electron waves Generally, control of such a magnetic field is difficult to implement. Also, the two paths shown in the article are formed vertically in a substrate, with a separation layer required at a branching portion of the paths, and this is difficult to implement from a manufacturing standpoint.
On the other hand, an arrangement using an electrostatic field to implement the Aharonov-Bohm effect in a semiconductor device has been described in articles by Bandyopadhyay et al in Superlattices and Microstructures, Vol. 2, No. 6, 1986 and in IEDM 86, pp. 76-79. In the Superlattices and Microstructures article, a generic ring structure for conducting electrodes, which is similar to FIG. 2 of the present specification is described by way of theoretical background However, because of practical difficulties in using such a theoretical conductor, both Bandyopadhyay et al articles describe an arrangement wherein a gate is used to apply an electrostatic field on two upper and lower conduction paths of GaAs which are separated from one another by an AlGaAs layer at a point where the two conduction paths pass under the gate. Because the upper path is closer to the gate, it will be subject to a stronger field from the gate. Therefore, a phase difference will exist between electrons travelling through the two paths at the point where the paths rejoin By virtue of this arrangement, Bandyopadhyay et al can modulate the drain current of the device based on the potential applied to the gate.
Although this electrostatic control arrangement in the Bandyopadhyay et al device is easier to control than the previously described magnetic arrangement, the implementation of the device is difficult because of the need to manufacture upper and lower channels separated from one another at a location under the gate. This requires a large number of difficult manufacturing steps, which make the device undesirable from a viewpoint of practical implementation.